Ribbon cables are used extensively in television, computer and related applications to carry many separate low power signals between adjacent physical components. The ribbon cable may have a generally flat configuration, formed by the generally side by side disposition of a plurality of separate electrical conductors or wires each separated from one another and surrounded by electrical insulation. Although the insulation is effective for electrically isolating the signals, it is ineffective in reducing radio frequency emmissions (RFI) and electromagnetic interference (EMI). As these interferences may disrupt the operation of the same or other adjacent components, such as in data transmission, FCC rules may call for shielding the ribbon cable.
An effective barrier for eliminating or reducing these interferences is provided by completely encircling the ribbon cable with a conductive material, such as copper or aluminum. The encircling conductive barrier can be extended axially along the ribbon cable, somewhat as a tube, and possibly even along its entire length including to the end connectors used to electrically and mechanically connect the ribbon cable to the respective physical components.
By way of example, the conductive barrier may be formed as a thin foil sheet, such as a 0.001 inch copper foil or a 0.002 inch aluminum foil. To provides added structural integrity to the conductive foil sheet and/or to minimize the possibility of the conductive sheet accidentally grounding any adjacent component, a thin sheet of MYLAR polyester insulating material or other insulating material may be bonded to or coated on one face of the conductive foil sheet (MYLAR is a trademark of the DuPont (El)deNemours Company). The insulating sheet may also be approximately 0.001 inch thick.
The encircling conductive sheet will typically be connected to a suitable electrical ground. This may be achieved by ground wire(s) connected, by solder, adhesive or the like, to the conductive sheet at spaced locations along the axial length of the ribbon cable. This may also be achieved by a pair of electrically conductive "drain" wires trapped between the conductive and insulating barrier sheets and extended axially along the ribbon cable adjacent the side edges of the ribbon cable.
This type of interference barrier will be referred to as an RF shield in this disclosure.
More specifically, an accessory type RF shield is the type intended to be put on in the field, by a tradesman-type installer; after the ribbon cable has been routed and connected between its adjacent physical components, and without the necessity of disconnecting the ribbon cable end connectors from the respective physical components.
Commercial accessory type RF shields thus may have the conductive and insulating barrier sheets in a generally flat original configuration sufficiently wide, side edge to side edge, to be able to be encircled around a ribbon cable with its opposite side edges overlapped as inner and outer layers. Means are provided on these overlapped side edges (at the inner and outer layers) to allow a field connection to be made for holding the barrier sheets in the encircling position around the ribbon cable.
To install such an RF shield, the conductive barrier sheet may be positioned against the ribbon cable, somewhat centered side edge to side edge, against one flat face of the ribbon cable; whereupon its opposite side edges can be folded around the ribbon cable to overlap as inner and outer layers against the opposite flat face of the ribbon cable, and to then be secured in place as overlapped.
In one commercial accessory type RF shield, an axially elongated adhesive strip is formed on the conductive sheet adjacent the side edge of the outer overlapped layer, and the insulating sheet in this region projects beyond the underlying side edge of the conductive sheet, to define a narrow axially extended insulating lip. As the outer layer is overlapped and pressed against the inner layer, the adhesive strip holds the overlapped layers together in this encircling position around the ribbon cable. The outer layer insulating lip overlies and possibly contacts the insulating layer on the inner layer, and thereby possibly covers the side edge of the outer layer conductive sheet, in an attempt to minimize interference leakage from this field connection.
However, several drawbacks exist by design in this type RF shield that reduce its effectiveness.
For example, the conductive sheet of the inner and outer layers do not ever contact one another providing less than an encircling conductive containment. This inherently allows some resulting interference leakage. Also, the outer layer insulating lip will start out spaced from the underlying inner layer insulating sheet by the combined thicknesses of the conductive sheet and the adhesive strip. The adhesive strip may be approximately 0.001 inch thick, or the same order of thickness of the barrier sheets. The outer layer insulating lip may be manually squeezed by the installer against the inner layer insulating sheet, but it is not mechanically held in this position. Thus, it will remain so positioned only by the material resilience of the insulating lip itself. When the outer layer insulating lip is spaced from the inner layer insulating sheet, the side edge of the outer layer conductive sheet remains exposed for potential interference leakage.
Another major drawback in this type RF shield relates to the manner of installing it in place on the ribbon cable. For example, in one embodiment of this RF shield, the folds of the barrier sheets must be determined solely by the installer; and the ease and accuracy of making the initial fold of the inner layer around the ribbon cable will be based significantly on the installer's skill and care. When shielding a long ribbon cable, the difficultly and potential for error increase dramatically. On the other hand, in another embodiment of this type RF shield, the drain wires are trapped between the barrier sheets almost precisely where the folds are to be made, that is at the opposite side edges of the ribbon cable, so that the barrier sheet folds typically will take place at these drain wires. While this helps folding the barrier sheets accurately, the overall effectiveness of the drain wires comes into question.
Thus, each drain wire is merely trapped between the barrier sheets, held in place by the mechanical bonding of the sheets in the regions immediately adjacent the wire. As the barrier sheets are folded at the drain wires, the inventors speculate that this folding action causes the inward collapse of the conductive foil sheet away from the wires, to reduce the soundness of the physical (and electrical) contact between the conductive sheet and drain wires. This may be the cause of the apparent reduced long-term effectiveness of RF shields of this type in providing a reliable ground.
Another commercial type of RF shield provides for a narrow section of the conductive foil sheet to be removed at the outer layer side edge, leaving a narrow axially extended lip of the insulating sheet; and a strip of adhesive is adhered to the inside face of this outer layer insulating lip. Conversely, a narrow section of the insulating sheet is removed from the inner layer side edge, leaving a narrow lip of the conductive sheet projecting therebeyond. The width sizing of this type RF shield is somewhat criticial to provide that when folded around the ribbon cable, the outer layer adhesive strip must line up over the inner layer insulating sheet and the inner layer conductive lip must underlie the outer layer conductive sheet. With this fit, the opposite edge portions of the conductive sheet should contact one another, to define an encircling conductive containment around the ribbon cable.
However, several drawbacks exist by design in this type RF shield, again reducing its effectiveness.
For example, as the barrier sheets and the adhesive strip may be of comparable thickness, and as the normal separation of the overlapped conductive sheet portions will be the combined thickness of the insulating sheet and the adhesive layer, a natural gap may be expected between the overlapped conductive sheet portions. Again, the manual squeezing action in installing the RF shield may cause these overlapped conductive sheet portions to contact one another; however they are not thereafter mechanically held together and will remain so positioned only by the material resilience of the conductive lip itself. Thus, from the start or after some use and time, the conductive sheet contact portions could gap to provide only a partial encircling conductive containment around the ribbon cable, with its inherent interference leakage.
As noted above, the successful utilization of these commercial RF shields is strongly dependent on the manner of locating them on and folding them around the ribbon cable. As the skill level of the installer is not a predictable or guaranteed factor, having RF shield designs that require extreme installing care and skill may not be favored. The previously mentioned types of commercial RF shields are provided in a flat state, side edge to side edge, so that both intermediate folds must be made around the side edges of the ribbon cable. This has been found to be a tremendous burden on the installer, particularly if the run of the ribbon cable is extended any significant distance such as might be measured in feet and large multiples thereof.